Explanation

The “only” that follows the blank indicates that the effect of the compound in water is an exception to its general state in the air. Since the compound is dangerous underwater, it follows that it is generally harmless in the air. The blank is preceded by “not,” so in order for the phrase to mean “harmless,” it must be filled by words that are synonymous with “harmful.” “Deleterious” and “detrimental” mean just that, and as synonyms they produce sentences alike in meaning, so they are the correct response. Of the other choices, “trivial” and “inconsequential” are synonyms, but they, like the other two choices, do not produce the necessary contrast with the last part of the sentence. Thus the correct answer is deleterious (Choice C) and detrimental (Choice E).
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why not F?